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Homework Help: Merge sort running time O(nlogn) formula

  1. Nov 1, 2015 #1
    If T denotes running time, then, as I heard at the lecture,

    T(n/2) + T(n/2) + O(n) = O(nlogn)

    where T(n/2) running time for sorting the left half
    T(n/2) running time for sorting the right half
    O(n) merging sorted

    Please, help me to see the math - how did the left side of expression turned into the right side one?

    Thank you!
  2. jcsd
  3. Nov 1, 2015 #2


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    2017 Award

    Staff: Mentor

    That requires T(n/2) to be O(n logn) and the equal sign is problematic.
    I think the underlying idea is: If you can combine two sets of n/2 elements within O(n) steps, the completely combining n sets of 1 element each to a single set of n elements requires O(n log n) steps because you get log(n) merging steps (lists of 1 -> lists of 2 -> lists of 4 -> ...)
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